The present invention relates to an image forming device incorporating image carriers for carrying toner images on their surfaces and transfer rollers for pressing a transfer medium against the image carriers to transfer the toner images carried by the image carriers to the transfer medium, and particularly to transfer roller used in such an image forming device.
Conventionally, in an image forming device incorporating an image carrier for carrying a toner image on its surface and a transfer roller for pressing the toner image carried by the image carrier against a transfer medium to transfer the toner image to the transfer medium, the surface of the transfer roller is made of elastic material, and the roller is fabricated in a so-called straight shape so that the diameter of a cross section taken at right angles to the axis is uniform over the entire length of the axis.
If the transfer roller has such a straight shape and the transfer medium is made of paper, such as a sheet of recording paper, partial whitening of the image may develop at the trailing end of the sheet depending on relationship between the direction of the fibers in the sheet and the transport direction of the sheet and other factors. The partial whitening at the trailing end of the sheet is likely to occur when the sheet has absorbed moisture in a non-uniform manner and is used in a highly humid environment. Besides, it is particularly likely to occur when the direction of the fibers in the sheet is not aligned with the transport direction of the sheet.
Specifically, if paper sheets as transfer media are stacked in a cassette and left in a highly humid environment for more than 15 hours, those near the bottom of the cassette are exposed to external air at their edges, but no so in the center. As a result, the edges absorb more moisture and grow thicker than the center, which increases the likelihood of partial whitening occurring along the edges. Those sheets that are left stacked near the top of the cassette are exposed to external air both at their edges and in their center and thereby absorb moisture uniformly. As a result, partial whitening at the trailing ends of the sheets is not likely to occur.
In addition, the transfer roller with a straight shape is likely to experience a higher transfer pressure in its axial end portions than in its axial midportion. Consequently, the toner image is not transferred in good conditions at the center of the recording paper, causing partial whitening.
Accordingly, Japanese Laid-Open Patent Application No. 4-22980/1992 (Tokukaihei 4-22980; published on Jan. 27, 1992) discloses a transfer roller of such a shape that the diameter of a cross section taken at right angles to an axis is smallest in the axial end portions and increases towards the axial midportion (hereinafter, will be referred to as a crown shape). Such a crown shape of the transfer roller causes the transfer pressure applied by the transfer roller to the image carrier to be greater in the axial midportion than in the axial end portions. This enables a suitable pressure to be applied to the recording paper sheet which is thicker at its edges than at its center due to non-uniform absorption of moisture at a press position (toner image transfer position) formed by the transfer roller and the image carrier. Thus, the toner image can be effectively transferred to the transfer medium, and partial whitening can be prevented.
Some image forming devices, like those disclosed in the aforementioned Japanese Laid-Open Patent Application, incorporate a pair of a transfer roller and an image carrier, while others incorporate pairs of them. The latter ones are typically called image forming devices of a tandem type.
In an image forming device of a tandem type, more than one image carrier are disposed parallel to one another in a transfer area where the toner image is transferred to a transfer medium, and an individual transfer roller, whose surface is made of elastic material, for pressing the toner image carried by the image carrier against the transfer medium to transfer the image to the transfer medium is provided for each of the image carriers.
The image forming device of a tandem type arranged as above is in some cases still falls short of effectively transferring the toner image to the transfer medium due to the non-uniform absorption of moisture by the transfer medium. To produce a better result in the transfer of the toner image to the transfer medium in an image forming device of a tandem type, one would naturally think of installing transfer rollers of a crown shape explained above.
However, in an image forming device of a tandem type, the transfer positions, i.e., the press sections formed by the image carriers and the transfer rollers, are lined along the transport path of the transfer medium, and toner images are transferred one by one to the transfer medium in associated transfer positions while it is moving along the path. Therefore, the resultant toner image formed on the transfer medium grows thicker as the transfer medium moves downstream along the path.
Accordingly, if those transfer rollers that are lined along the transport path share an identical difference between the diameters of cross sections taken at right angles to an axis in the axial midportion and in the axial end portions (hereinafter, will be referred to as a crown quantity), and the transfer roller located in the most upstream transfer position is adjusted so that it would exert a substantially identical transfer pressure to the image carrier in its axial midportion and in its axial end portions, the remaining transfer rollers, located in relatively downstream positions, exert different transfer pressures in the axial midportion and in the axial end portions due to the thickness of the toner image and fail to transfer the toner image in more stable conditions.
Alternatively, if the transfer roller located in the most downstream transfer position is adjusted so that it would exert a substantially identical transfer pressure to the image carrier in its axial midportion and in its axial end portions, the remaining transfer rollers, located in relatively upstream positions, can only exert an overall transfer pressure that is insufficient due to a smaller thickness of the toner image formed on the transfer medium in those positions than in the most downstream position and fail to transfer to toner image in stable conditions.
Further, due to the shape of the transfer rollers, it is extremely difficult to cause the transfer pressure exerted by the transfer roller in every transfer position to be adjustable along the axis of the transfer roller and thereby enable the transfer roller to exert an identical transfer pressure to the image carrier in its axial midportion and in its axial end portions.
As detailed so far, the transfer rollers of a crown shape, when used in the image forming device of a tandem type, develop unstable transfer conditions either in upstream transfer positions or downstream transfer positions and result in a problem that a satisfactory copy image cannot be formed on the transfer medium.
The present invention has an object to provide an image forming device capable of forming a clear and vivid copy image on a transfer medium.
To accomplish the object, an image forming device in accordance with the present invention includes:
image carriers provided parallel to each other in a transfer area where toner images are transferred to a transfer medium; and
transfer rollers, each being associated to a different one of the image carriers, for pressing and thus transferring the toner images carried by the image carriers to the transfer medium,
wherein:
each of the transfer rollers is fabricated in a crown shape and so that a difference between diameters of cross sections taken at right angles to an axis in an axial midportion and in axial end portions decreases in stages towards a downstream side along a transport direction of the transfer medium.
With the arrangement, the transfer rollers fabricated in a crown shape; therefore the transfer pressure exerted by the transfer roller to the image carrier can be prevented from decreasing in the axial midportions of the transfer positions (press sections where the transfer roller press the image carriers). As a result, the transfer pressure exerted to the transfer medium transported to the transfer position can be rendered substantially uniform in the axial midportion and in the axial end portions of the transfer roller.
In addition, the difference between the diameters of cross sections taken at right angles to an axis in the axial midportion and in the axial end portions of the transfer roller, i.e., the crown quantity, decreases in stages along the transport direction of the transfer medium; therefore, the transfer pressure exerted to the image carrier in the axial midportion of the transfer roller can also be caused to decrease in stages along the transport direction of the transfer medium. This allows the transfer pressure on the transfer medium on which the toner layer grows thicker in stages towards the downstream side along the transport direction of the transfer medium to be reduced in stages in the axial midportions of the transfer positions and thereby enables the transfer pressure on the transfer medium in the transfer positions to be substantially uniform in the axial midportion and in the axial end portions of the transfer roller.
As a result, a clear and vivid, well-fixed copy image can be formed on the transfer medium with no partial whitening both in the upstream and downstream transfer positions along the transport direction.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.